1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus, and more particularly, to an imaging lens applicable for, for example, an in-vehicle camera, a mobile terminal camera, or a monitoring camera using an imaging device, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor), and an imaging apparatus including the imaging lens.
2. Description of the Related Art
In recent years, the size of an imaging device, such as a CCD or a CMOS, has been reduced, and the number of pixels thereof has increased. In addition, the size of an imaging apparatus including the imaging device has been reduced. Therefore, it is necessary to improve the optical performance of an imaging lens provided in the imaging apparatus and reduce the size and weight of the imaging lens.
Meanwhile, for example, an in-vehicle camera or a monitoring camera requires a high-performance lens that has high weather resistance, can be used in a wide temperature range in a vehicle from a low temperature in a cold region to a high temperature in a tropical region, and has such a small F number that it can be used in the night.
Japanese Patent Application Nos. 2007-236445 and 200 8-193299 (which correspond to US 2009/0067063) and Japanese Patent Application No. 2008-59127 filed by the inventors disclose a lens system having six lenses including a cemented lens as an imaging lens that can be used in the above-mentioned field. In addition, Japanese Patent No. 3723654, JP-A-2005-24969, JP-A-11-142730, Japanese Patent No. 3723637, and Japanese Patent No. 3478643 disclose imaging lenses that can be used in the above-mentioned field. Japanese Patent No. 3723654 discloses an imaging lens including five lenses, that is, negative, positive, negative, positive, and positive lenses arranged in this order. JP-A-2005-24969 and JP-A-11-142730 disclose an imaging lens including six lenses that are classified into a front group including an aspheric lens and a rear group having a positive refractive power. Japanese Patent No. 3723637 discloses an imaging lens that includes six lenses and has a large back focal length. Japanese Patent No. 3478643 discloses an imaging lens including six lenses in which a cemented lens is arranged closest to an image side.
However, the imaging lens disclosed in Japanese Patent No. 3723654 includes five lenses, and the aberration of the imaging lens needs to be reduced, as compared to an imaging lens having six lenses.
The imaging lens disclosed in JP-A-2005-24969 and JP-A-11-142730 use an aspheric lens. However, when the aspheric lens is made of resin, the performance of the imaging lens is likely to be lowered due to a temperature variation. Therefore, when the imaging lens is used in a wide temperature range, problems may arise. For this reason, it is preferable that the aspheric lens be made of glass. However, when the aspheric lens is made of glass, a glass mold is needed to manufacture the aspheric lens, which results in an increase in manufacturing costs.
The imaging lenses disclosed in Japanese Patent No. 3723637 and Japanese Patent No. 3478643 use only the glass spherical lens. Therefore, the imaging lens is cheaper than a structure using a glass mold aspheric lens. However, since the imaging lens disclosed in Japanese Patent No. 3723637 has a large total length, sufficient reduction in the size of the imaging lens is not achieved. The imaging lens disclosed in Japanese Patent No. 3478643 has a relatively small size, but the F number thereof is 2.8. Therefore, the imaging system is not fast enough to be used for an in-vehicle camera or a monitoring camera.
Meanwhile, in the above-mentioned imaging apparatuses, in some cases, a ghost is generated in an image surface due to light reflection from each lens surface or the imaging surface of the CCD. Therefore, there is a concern that it may be difficult to exactly recognize an image according to the degree of ghost. In particular, it is necessary to prevent the generation of the ghost in an imaging apparatus, such as a monitoring camera or an in-vehicle camera that captures the image of the front side and performs image processing.